Microporous hollow fiber membranes from perfluorinated thermoplastic polymers

ABSTRACT

High flux porous hollow fiber membranes are produced from perfluorinated thermoplastic polymers by extruding a heated solution of the polymer having a lower critical solution temperature directly into a cooling bath to form the porous membrane by liquid-liquid phase separation. Extrusion can be conducted either vertically or horizontally.

This application claims the benefit of Provisional Application No.60/117,852, filed Jan. 29, 1999.

This invention relates to a process to produce hollow fiber porousmembranes from perfluorinated thermoplastic polymers. More specifically,this invention relates to a process to produce microporous membraneshaving an essentially skin-free surface on at least one of the inner andouter surfaces, and to the membranes produced.

BACKGROUND OF THE INVENTION

Microporous membranes are used in a wide variety of applications. Usedas separating filters, they remove particles and bacteria from diversesolutions such as buffers and therapeutic containing solutions in thepharmaceutical industry, ultrapure aqueous and organic solvent solutionsin microelectronics wafer making processes, and for pre-treatment ofwater purification processes. In addition, they are used in medicaldiagnostic devices, where their high porosity results in advantageousabsorption and wicking properties.

Hollow fiber membranes are also used as membrane contactors, typicallyfor degassing or gas absorption applications. Contactors bring togethertwo phases, i.e., two liquid phases, or a liquid and a gas phase for thepurpose of transferring a component from one phase to the other. Acommon process is gas-liquid mass transfer, such as gas absorption, inwhich a gas or a component of a gas stream is absorbed in a liquid.Liquid degassing is another example, in which a liquid containingdissolved gas is contacted with an atmosphere, a vacuum or a separatephase to remove the dissolved gas. In an example of conventional gasabsorption, gas bubbles are dispersed in an absorbing liquid to increasethe gas/liquid surface area and increase the rate of transfer of thespecies to be absorbed from the gas phase. Conversely, droplets ofliquid can be sprayed or the liquid can be transported as a thin film incounter-current operation of spray towers, packed towers, etc.Similarly, droplets of an immiscible liquid can be dispersed in a secondliquid to enhance transfer. Packed columns and tray columns have adeficiency as the individual rates of the two phases cannot beindependently varied over wide ranges without causing flooding,entrainment, etc. If however, the phases are separated by a membrane,the flow rates of each phase can be varied independently. Furthermore,all the area is available, even at relatively low flow rates. Due tothese advantages, hollow fiber membranes are increasingly being used incontactor applications.

Hydrophobic microporous membranes are commonly used for contactorapplications with an aqueous solution that does not wet the membrane.The solution flows on one side of the membrane and a gas mixture at alower pressure than the solution flows on the other. Pressures on eachside of the membrane are maintained so that the liquid pressure does notovercome the critical pressure of the membrane, and so that the gas doesnot bubble into the liquid. Critical pressure, the pressure at which thesolution will intrude into the pores, depends directly on the materialused to make the membrane, inversely on the pore size of the membrane,and directly on the surface tension of the liquid in contact with thegas phase. Hollow fiber membranes are primarily used because of theability to obtain a very high packing density with such devices. Packingdensity relates to the amount of useful filtering surface per volume ofthe device. Also, they may be operated with the feed contacting theinside or the outside surface, depending on which is more advantageousin the particular application. Typical applications for contactingmembrane systems are to remove dissolved gases from liquids,“degassing”; or to add a gaseous substance to a liquid. For example,ozone is added to very pure water to wash semiconductor wafers.

Porous contactor membranes are preferred for many applications becausethey will have higher mass transfer than nonporous membranes. Forapplications with liquids having low surface tensions, smaller poresizes will be able to operate at higher pressures due to theirresistance to intrusion. For applications in which the gas to betransferred in highly soluble in the liquid phase, the mass transferresistance of skinned membranes is a detriment to efficient operation.

Z. Qi and E. L. Cussler (J. Membrane Sci. 23(1985) 333-345) show thatmembrane resistance controls absorption of gases such as ammonia, SO₂and H₂S in sodium hydroxide solutions. This seems generally true forcontactors used with strong acids and bases as the absorption liquid.For these applications, a more porous contactor membrane, such as amicroporous membrane, would have an advantage, because the membraneresistance would be reduced. This would be practical if the liquid doesnot intrude the pores and increase resistance. With the very low surfacetension materials used in the present invention, this would be possiblewithout coating the surface of the fibers with a low surface tensionmaterial, which is an added and complex manufacturing process step.

An advantage for contacting applications is that the very low surfacetension of these perfluorinated polymers allows use with low surfacetension liquids. For example, highly corrosive developers used in thesemiconductor manufacturing industry may contain surface tensionreducing additives, such as surfactants. These developers could not bedegassed with typical microporous membranes because the liquid wouldintrude the pores at the pressures used and permeate, causing solutionloss and excess evaporation. In addition, liquid filling the pores wouldgreatly add to the mass transfer resistance of gas transport. U.S. Pat.No. 5,749,941 describes how conventional hollow fiber membranes ofpolypropylene or polyethylene cannot be used in carbon dioxide orhydrogen sulfide absorption into aqueous solutions containing an organicsolvent without the use of a solution additive to prevent leakage. WhilePTFE membranes would work in these applications, presumably because oftheir lower surface tension, they are difficult to process into hollowfibers. The membranes of the present invention are made from polymershaving similar surface tension properties to PTFE and are more readilymanufactured into small diameter hollow fiber membranes.

Microporous membranes have a continuous porous structure that extendsthroughout the membrane. Workers in the field consider the range of porewidths to be from approximately 0.05 micron to approximately 10.0microns. Such membranes can be in the form of sheets, tubes, or hollowfibers. Hollow fibers have the advantages of being able to beincorporated into separating devices at high packing densities. Packingdensity relates to the amount of useful filtering surface per volume ofthe device. Also, they may be operated with the feed contacting theinside or the outside surface, depending on which is more advantageousin the particular application.

A hollow fiber porous membrane is a tubular filament comprising an outerdiameter, an inner diameter, with a porous wall thickness between them.The inner diameter defines the hollow portion of the fiber and is usedto carry fluid, either the feed stream to be filtered through the porouswall, or the permeate if the filtering is done from the outer surface.The inner hollow portion is sometimes called the lumen.

The outer or inner surface of a hollow fiber microporous membrane can beskinned or unskinned. A skin is a thin dense surface layer integral withthe substructure of the membrane. In skinned membranes, the majorportion of resistance to flow through the membrane resides in the thinskin. In microporous membranes, the surface skin contains pores leadingto the continuous porous structure of the substructure. For skinnedmicroporous membranes, the pores represent a minor fraction of thesurface area. An unskinned membrane will be porous over the majorportion of the surface. The porosity may be comprised of single pores orareas of porosity. Porosity here refers to surface porosity, which isdefined as the ratio of surface area comprised of the pore openings tothe total frontal surface area of the membrane. Microporous membranesmay be classified as symmetric or asymmetric, referring to theuniformity of the pore size across the thickness of the membrane. In thecase of a hollow fiber, this is the porous wall of the fiber. Symmetricmembranes have essentially uniform pore size across the membranecross-section. Asymmetric membranes have a structure in which the poresize is a function of location through the cross-section. Another mannerof defining asymmetry is the ratio of pore sizes on one surface to thoseon the opposite surface.

Manufacturers produce microporous membranes from a variety of materials,the most general class being synthetic polymers. An important class ofsynthetic polymers are thermoplastic polymers, which can be flowed andmolded when heated and recover their original solid properties whencooled. As the conditions of the application to which the membrane isbeing used become more severe, the materials that can be used becomeslimited. For example, the organic solvent-based solutions used for wafercoating in the microelectronics industry will dissolve or swell andweaken most common polymeric membranes. The high temperature strippingbaths in the same industry consist of highly acid and oxidativecompounds, which will destroy membranes made of common polymers. Sincemembranes made from perfluorinated thermoplastic polymers such aspoly(tetrafluoroethylene-co-perfluoro(alkylvinylether))(POLY(PTFE-CO-PFVAE)) orpoly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) are not adverselyaffected by severe conditions of use, they have a decided advantage overmembranes made from less chemically and thermally stable polymers.

Being chemically inert, the POLY(PTFE-CO-PFVAE) and FEP polymers aredifficult to form into membranes using typical solution casting methods.They can be made into membranes using the Thermally Induced PhaseSeparation (TIPS) process. In one example of the TIPS process, a polymerand organic liquid are mixed and heated in an extruder to a temperatureat which the polymer dissolves. A membrane is shaped by extrusionthrough an extrusion die, and the extruded membrane is cooled to form agel. During cooling the polymer solution temperature is reduced to belowthe upper critical solution temperature. This is the temperature at orbelow which two phases form from the homogeneous heated solution, onephase primarily polymer, the other primarily solvent. If done properly,the solvent rich phase forms a continuous interconnecting porosity. Thesolvent rich phase is then extracted and the membrane dried.POLY(PTFE-CO-PFVAE) and FEP membranes made by the TIPS process aredisclosed in U.S. Pat. No. 4,902,456, 4,906,377; 4,990,294; and5,032,274. In the U.S. Pat. Nos. 4,902,456 and 4,906,377 patents, themembranes have a dense surface with either intervals of crack-likeopenings or pores, either singly, or as a series of several pores. TheU.S. Pat. Nos. 4,990,294 and 5,032,274 patents disclose using a coatingof the dissolution solvent on the shaped membrane as it exits the die.Both surfaces consist of a dense skin with porous areas. In oneembodiment, membrane produced without co-extrusion in a sheet form isstretched in the transverse direction. The membrane surface for thesemembranes consists of nodular appearing structures separated bycrack-like openings.

U.S. Pat. No. 5,395,570 discloses a method of extrusion of hollow fibermembranes in which a quadruple extrusion head is used to extrude ahollow fiber with a lumen-filling fluid, a coating layer, and a coolingfluid layer. This method requires a complex extrusion head and flowcontrol means, and a separate coating layer consisting of the solventbetween the cooling fluid and the extruded fiber. Also, the extrudedfiber is not immediately contacted with the cooling fluid, but passes toa lower zone of the extrusion head before the fourth (cooling) layer iscontacted with the coated fiber.

U.S. Pat. No. 4,564,488 discloses a process for preparing porous fibersand membranes. The process involves forming a homogeneous mixture of apolymer and at least another liquid inert with respect to the polymer.The mixture must have a temperature range of complete miscibility and atemperature range where there is a miscibility gap. The mixture isextruded at a temperature above the separation temperature into a bathpreferably containing entirely or for the most part the inert liquid.The bath is maintained below the separation temperature. Disclosed butnot claimed is an embodiment wherein the homogeneous mixture is extrudedimmediately into the bath containing entirely or for the most part theinert liquid, i.e., solvent No perfluorinated thermoplastic polymers arelisted as “customary polymers” that are in the scope of the patent. Nomention is made of special methods needed to extrude immediately intothe cooling bath at very high temperatures.

WO 95/02447 discloses asymmetric PTFE membranes made by coating asolution of PTFE in a perfluorinated cycloalkane heated to about 340° C.onto a substrate, removing the solvent and cooling the PTFE on thesubstrate so that one surface of the membrane is less porous than theother, and optionally, removing the substrate. No mention is made ofapplying this method to unsupported hollow fiber membranes.

U.S. Pat. No. 4,443,116 discloses a process for making a porousfluorinated polymer structure. Applicable polymers are copolymers oftetrafluoroethylene and perfluorovinylether with a sulfonyl fluoride(—SO₂F), sulfonate (SO₃Z) or carboxylate (COOZ) functional group whereinZ is a cation. The presence of the polar functional group greatlyfacilitates solubility. A thermally induced phase separation method isused in which the solvent must crystallize after cooling and phaseseparation. The solvent is removed while in a solid state. No porestructure or permeability data are given.

PTFE, POLY(PTFE-CO-PFVAE) and FEP sheet membranes are disclosed in U.S.Pat. No. 5,158,680, wherein an aqueous dispersion of PTFE with particles1 micron or less and a filament forming polymer are mixed, formed into amembrane shape and heated to above the melting temperature and then thefilament forming polymer is removed.

For filtration of ultrapure solutions, vanishingly low levels ofextractable residual matter is required of the membrane. The TIPSprocess requires only the removal of the low molecular weight extrusionsolvent after extrusion. This material is easily removed by extractionwith a solvent, and since the POLY(PTFE-CO-PFVAE) and FEP material isinert to the extraction solvent, no change of membrane propertiesoccurs. Extraction is simple and thorough due to the high porosity ofthe membrane and the high diffusion of the low molecular weight solvent.Membranes made by extraction of a polymer or resin would meet theserequirements with extreme difficulty due to the inherent difficulty ofremoving the slowly diffusing polymers or resins.

Previous POLY(PTFE-CO-PFVAE) and FEP membranes made from the TIPS methodrequired extrusion through an air gap. POLY(PTFE-CO-PFVAE) and FEPmembranes made by the TIPS process are disclosed in U.S. Pat. Nos.4,902,45; 4,906,377: 4,990,294; and 5,032,274. In the U.S. Pat. Nos.4,902,456 and 4,906,377 patents, the membranes have a dense surface witheither intervals of crack-like openings or pores, either singly, or as aseries of several pores. The U.S. Pat. Nos. 4,990,294, and 5,032,274patents disclose using a coating of the dissolution solvent on theshaped membrane as it exits the die. In one embodiment, the membrane ina sheet form is stretched in the transverse direction. It was found thatthe rapid evaporation of the solvent at the high extrusion temperaturesgave skinning and poor control of the surface porosity. To overcome theskinning problems, a solvent coating method and post-stretching wereemployed by previous inventors. In the solvent coating method, thesolvent, hot Halocarbon oil, heated to around 300° C., is used to coatthe melt surfaces as soon as the melt emerges from the die. While thismethod does suppress evaporation, it introduces other processingproblems. First, it is very difficult to coat a melt surface uniformlywith hot solvent because hot Halocarbon oil has the tendency to formdroplets. Instead of a uniform coating, the solvent coating tends tostreak along the melt surface. After the solution is cooled andsolidified, the membrane surface shows uneven porosity due tonon-uniform coating of solvent. Second, the temperature of the oil maynot be uniform, and the resulting membrane would show high degree ofvariation of membrane properties due to uneven quenching of thesurfaces. Third, the hot oil tends to soften the extruded melt and theextruded fiber tends to break apart during processing.

Post-stretching was disclosed as another technique to enhancepermeability of a skinned PFA membrane in U.S. Pat. Nos. 4,990,294, and5,032,274. While stretching does increase permeability substantially, itproduces its own set of undesirable side effects. First, for stretchingto be effective, the base skinned membrane must be very uniform inthickness and in mechanical strength. Any non-uniformity in the basemembrane will be amplified as soon as the membrane is subjected tostretching, because weak areas stretch more than strong areas under thesame stretching force. As mentioned above, it is very difficult toproduce base membranes with the solvent coating technique. If solventcoating is not used, the heavy evaporation of porogen usually producesdried polymer on the die lips. This accumulated dried polymer thenscratches the melt surfaces, producing lines of hidden weaknesses in thebase membrane. Upon stretching, the weakened membranes break apart alongthe “scratch” lines.

It would therefore be desirable to have a process that would eliminatethe rapid evaporation of solvent from the fiber surface, but not requirea difficult coating or stretching step. It would also be beneficial toproduce a skinless membrane having high surface porosity in order toutilize a large proportion of the membrane surface for permeation andretention.

It would further be desirable to have a porous hollow fiber contactormembrane for applications in which a highly soluble gas is to betransferred to a liquid having low interfacial tension.

SUMMARY OF THE INVENTION

This invention provides for high flux, skin-free hollow fiber porousmembranes, more specifically, microporous membranes, from perfluorinatedthermoplastic polymers, more specificallypoly(tetrafluoroethylene-co-perfluoro(alkylvinylether))(POLY(PTFE-CO-PFVAE)) orpoly(tetrafluoroethylene-co-hexafluoropropylene) (FEP). These membranesare capable of operating in severe chemical environments with noapparent extractable matter being released. Compared to prior artmembranes, the membranes of the invention have a higher surfaceporosity, which translates into high permeability or flux.

An embodiment of this invention provides for porous hollow fibercontactor membranes from perfluorinated thermoplastic polymers, morespecifically poly(tetrafluoroethylene-co-perfluoro(alkylvinylether))(POLY(PTFE-CO-PFVAE)) orpoly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), and their use.

A process to produce these membranes is provided. The process is basedon the Thermally Induced Phase Separation (TIPS) method of making porousstructures and membranes. A mixture of polymer pellets, usually groundto a size smaller than supplied by the manufacturer, and a solvent, suchas chlorotrifluoroethylene oligomer, is first mixed to a paste orpaste-like consistency. The polymer comprises between approximately 12%to 35% by weight of the mixture. The solvent is chosen so the membraneformation occurs by liquid-liquid, rather than solid-liquid phaseseparation when the solution is extruded and cooled. Preferred solventsare saturated low molecular weight polymers of chlorotrifluoroethylene.A preferred solvent is HaloVac® 60 from Halocarbon Products Corporation,River edge, N.J. Choice of the solvent is dictated by the ability of thesolvent to dissolve the polymer when heated to form an upper criticalsolution temperature solution, but not to excessively boil at thattemperature. Fiber extrusion is referred to as spinning and the extrudedfiber length from the die exit to the take-up station is referred to asthe spin line. The paste is metered into a heated extruder barrel wherethe temperature raised to above the upper critical solution temperatureso that dissolution occurs. The homogeneous solution is then extrudedthrough an annular die directly into a liquid cooling bath with no airgap. The liquid cooling bath is maintained at a temperature below theupper critical solution temperature of the polymer solution. Thepreferred bath liquid is not a solvent for the thermoplastic polymer,even at the extrusion temperature. Upon cooling, the heated and shapedsolution undergoes phase separation and a gel fiber results. The die tipis slightly submerged for vertical spinning, i.e.; the spin line fallsdownward, in the direction of a freely failing body. For horizontalspinning, where the spin line exits directly in the horizontal attitude,and is maintained more or less in that plane until at least the firstguide roll, a specially design die is used. The die is firmly positionedagainst an insulated wall with the die tip penetrating through anopening having a liquid-tight seal in the insulator wall. A trough forcooling liquid flow is placed in a recess in the opposite side of theinsulating wall, in a manner that will maintain the die nose outlet in asubmerged condition. Cooling liquid flows in the trough and overflows ina region of the trough of lesser depth, keeping the die nose outletsubmerged with a flow of cooling liquid. In both the vertical andhorizontal methods, a booster heater and temperature control means isused to briefly raise the solution temperature at the die tip to preventpremature cooling. In a subsequent step, the dissolution solvent isremoved by extraction and the resultant hollow fiber membrane is driedunder restraint to prevent membrane shrinkage and collapse. Optionallythe dried fiber may be heat set at 200° C. to 300° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of the process of this invention with verticalextrusion.

FIG. 2 is a flow diagram of the process of this invention withhorizontal extrusion.

FIG. 3 is a drawing of the die used in vertical fiber spinning.

FIG. 4 is a drawing of the die used in horizontal fiber spinning.

FIG. 5 is a photomicrograph at 3191× of the inner surface of a hollowfiber microporous membrane made frompoly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) made inaccordance with Example 1, Sample #3.

FIG. 6 is a photomicrograph at 3191× of the outer surface of a hollowfiber microporous membrane made frompoly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) made inaccordance with Example 1, Sample #3.

FIG. 7 is a photomicrograph at 3395× of the inner surface of a hollowfiber microporous membrane made frompoly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) made inaccordance with Example 1, Sample #8

FIG. 8 is a photomicrograph at 3372× of the outer surface of a hollowfiber microporous membrane made frompoly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) made inaccordance with Example 1, Sample #8.

FIG. 9 is a photomicrograph at 984× of the inner surface of a hollowfiber microporous membrane made frompoly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), made inaccordance with Example 5.

FIG. 10 is a photomicrograph at 1611× of the outer surface of a hollowfiber microporous membrane made frompoly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), made inaccordance with Example 5.

FIG. 11 is a chart comparing the performance of hollow fiber membranecontactors made from skinned membranes to a contactor made fromunskinned membranes in water ozonation.

FIG. 12 is a schematic of the test stand used to compare contactors influid-fluid contacting applications.

FIG. 13 is a chart comparing absorption of carbon dioxide in water usingcontactors with skinned membranes and unskinned membranes.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

A person of ordinary skill in the art of making porous membranes willfind it possible to use the teachings of the present invention toproduce essentially skin-free hollow fiber porous membranes fromperfluorinated thermoplastic polymers which can be dissolved in asolvent to give a solution having an upper critical solutiontemperature, and which when the solution is cooled, separates into twophases by liquid-liquid phase separation. Examples of such polymers arepoly(tetrafluoroethylene-co-perfluoro(alkylvinylether))(POLY(PTFE-CO-PFVAE)) orpoly(tetrafluoroethylene-co-hexafluoropropylene) (FEP). PFA Teflon® isan example of a poly(tetrafluoroethylene-co-perfluoro(alkylvinylether))in which the alkyl is primarily or completely the propyl group. FEPTeflon® is an example ofpoly(tetrafluoroethylene-co-hexafluoropropylene). Both are manufacturedby DuPont. Neoflon™ PFA (Daikin Industries) is a polymer similar toDuPont's PFA Teflon®. Apoly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) polymer in whichthe alkyl group is primarily methyl is described in U.S. Pat. No.5,463,006. A preferred polymer is Hyflon® POLY(PTFE-CO-PFVAE) 620,obtainable from Ausimont USA, Inc., Thorofare, N.J.

With the POLY(PTFE-CO-PFVAE), PFA, and FEP polymers, saturated lowmolecular weight polymers of chlorotrifluoroethylene have been found tobe useful solvents. A preferred solvent is HaloVac® 60, HalocarbonProducts Corporation, River Edge, N.J.

Fiber Spinning Compositions

A paste of polymer and solvent is made by mixing the desired amount ofweighed solvent to pre-weighed polymer in a container. The polymer haseither been obtained in a particle size of approximately 100 to 1000micron size, preferably about 300 micron size, or previously reduced tothat size range by a suitable grinding process. Larger size particles donot completely dissolve in the preferred heating step, requiringadditional heating time, and smaller particles require more expensivegrinding which increases the cost of the process. The polymer comprisesbetween approximately 12% to 35% of the mixture. Mixtures aboveapproximately 35% do not give suitable porosity, and at belowapproximately 12% polymer content, the resulting fibers are too weak.

An example of a saturated low molecular weight polymers ofchlorotrifluoroethylene is HaloVac 60. (Halocarbon ProductsCorporation). Choice of the solvent is dictated by the ability of thesolvent to dissolve the polymer when heated to form an upper criticalsolution temperature solution, but not to excessively boil at thattemperature. When dissolution takes place at a temperature well abovethe boiling point of the solvent, bubbles form in the extrudate andcause spin line breakage. The solvent need not be a single purecompound, but may be a blend of molecular weights, or copolymer ratios,of low molecular weight polymers of chlorotrifluoroethylene. Such blendscan be adapted to balance solubility with suitable boiling pointcharacteristics.

Dissolution and Extrusion

The paste is metered into the heated mixing zone of a conventional twinscrew extruder and heated to a preferred temperature of about 270° C. toabout 320° C., with a more preferred range of 285° C. to 310° C.,preferably under an minert atmosphere, such as nitrogen, to preventdegradation of the solvent at these temperatures. The temperature isdependent on the melting temperature of the polymer being used. Theextruder conveys the heated solution to an in-line heated metering pump,which feeds the solution to the annular die and controls the rate ofextrusion. Optional inline filters can be used, if required.

Fiber Extrusion

Hollow fiber membrane production presents difficulties not encounteredwith membrane production such as sheet membrane where the membrane issupported as is solidifies. In the case of hollow fiber production atvery high temperatures, these problems are magnified. Hollow fibers aremade by extruding a polymer solution or dispersion through the annularspace of a die made of two concentric tubes. The inner tube carries afluid or gas, which maintains the inner diameter defining the lumenduring solidification. In operation, the polymer solution is co-extrudedwith the lumen fluid into a liquid bath. In the thermally induced phaseseparation method of this invention, the bath liquid is maintained at atemperature below which phase separation occurs for the polymer solutionbeing used. The shaped solution cools, phase separation takes place, andthe fiber solidifies. Unlike flat sheet membranes, which are coated orextruded onto a roll or a web carrier, or tubular membranes, which areformed on the inner or outer surface of a mandrel, extruded hollowfibers are not supported while they are solidifying. Since the extrudedsolution is not supported, the forces that transport the fiber throughthe cooling bath are operating directly on the shaped solution as itsolidifies. If too large, the forces will pull the fiber apart.

For the fibers of the present invention, there are two inter-relatedproblems that had to be overcome in order to have a useful process.These are the need to have a skinless membrane for high permeability,and to be able to extrude a solution that would have sufficient strengthto be continuously produced at a practical rate. Perfluorinatedthermoplastics melt at high temperatures, approximately 260° C.-300° C.,and are difficult to dissolve. Few solvents are known and even thesaturated low molecular weight polymers of chlorotrifluoroethylene founduseful have limitations. For these solvents, higher molecular weightspecies have higher boiling points. It is commonly accepted that in aTIPS process that the boiling point of the solvent should be greaterthan the polymer melting temperature by 25° C.-100° C. and should have alow volatility at the extrusion temperature. (Lloyd, D. R. et al, J.Membrane Sci. 64 1-11 (1991)). However, saturated low molecular weightpolymers of chlorotrifluoroethylene with boiling points greater thanabout 280° C. are not good solvents for these polymers. Therefore, amethod had to be developed to use solvents having boiling points loweror near to the melting temperature of the polymer. At thesetemperatures, the solvent is very volatile and if an air gap is used,rapid loss of solvent in the air gap will increase the polymerconcentration at the fiber surface and result in a skinned membrane withlow permeability. To prevent skin formation from rapid evaporation ofthe solvent, the die outlet is submerged in the cooling bath.

Submerged extrusion, although seemingly simple, is actually verydifficult to achieve practically. In the TIPS process, heated extrudatepasses through an air gap before contacting the cooling surface or bathliquid. The air gap, the distance from the outlet of the die to thecooling or quenching surface, serves the very important function ofallowing the melt to draw. Draw can be described by the ratio of themembrane wall thickness to the annular space of the die. The air gapallows the melt to accelerate (drawing) and to be taken up at a high andeconomical rate. For hollow fiber submerged extrusion, however, only alow draw ratio can be tolerated because the extruded fiber rapidly coolsand solidifies as it exits the die into the cooling bath, and becomesresistant to drawing. Not being fully solidified, the fiber has a strongtendency to break. Therefore, it is necessary to spin the fibers with alow draw ratio.

In this invention, submerged extrusion was perfected to eliminate theair gap. First, to sidestep the drawing dilemma, a hollow fiber die wasmade with an unusually narrow die gap of about 350-400μ, which definesthe wall thickness. This is very close to the dimension of the finalfiber so that minimal drawing is required. The die was designed andmachined so that only the tip, about {fraction (1/16)} of an inch, madecontact with the quench liquid. This modification is crucial to thesuccess of this technique. Since the quench liquid has a much lowertemperature than the die body, submerging a conventional die would dropthe temperature of the die to the point that the solution loses itsability to flow. Even with just the tip submerged, there was a decreasein the temperature of the die tip. A microthermocouple and astrategically located booster heater were used to control thetemperature of the die tip and to raise the solution temperature at thedie tip.

Fiber can be extruded in either of two attitudes, horizontally orvertically, as shown in FIGS. 1 and 2. The solution is metered throughthe annular die by the metering pump at a volumetric rate thatapproximately matches the take up rate of the spin line. This isnecessary to prevent any significant drawdown of the fiber which willcause breakage of the weak extrudate. The inner and outer diameter, andthe resulting annular space are set by the requirements for the finalfiber. A wall thickness of from 100 microns to 250 microns, preferably150 microns to 200 microns, will give a useful fiber. Spin line take-uprates are dependent on the fiber dimensions and extrusion rate. Rates offrom approximately 20 to approximately 200 feet per minute can be used,with a preferred rate being approximately 100 feet per minute.

During fiber extrusion the inner diameter of the die is filled with acontinuous flow of liquid to prevent the fiber lumen from collapsing.Careful control of the lumen liquid flow rate is required to preventuncontrolled variations in fiber dimensions. The liquid should have aboiling point high enough so that boiling will not occur in the die orthe extruded fiber. This can cause bubbles in the lumen and fiberbreakage. The lumen liquid should not affect the fiber inner wall in away that will cause the inner surface to densify. As, for example, bycausing coagulation of the heated solution at the lumen liquid-innerwall contact interface, or by extracting solvent from this interface andincreasing the surface polymer concentration. The lumen liquid can bemetered into the die at room temperature, or preheated to a temperatureof up to 200° C.

The die is comprised of a standard cross-head die, to which is attacheda die nose. The die has two temperature control zones. The crossheadportion of the die is kept at 270° C. to 320° C., with preferredtemperature range being 280° C. to 290° C. The die nose, whichencompasses the die outlet, is controlled separately to a range of 290°C. to 320° C., preferably to 300° C. to 310° C. The die nose heated zonebriefly raises the solution temperature to near or above the boilingpoint of the solvent.

FIG. 1 illustrates the die nose used for vertical fiber spinning. Thesolution is introduced to circular inlet 3 from the cross-head die andis transported to die exit 9. Lumen fluid is introduced to the die noseat inlet 2, and exits at the die exit. Heater 5 maintains the solutionin a fluid form. Temperature sensor 6 is used with a temperaturecontroller to maintain heater 5 at a determined temperature above theseparation temperature of the solution. Die tip 9 is submerged incooling bath 7. Gel membrane hollow fiber 8 exits the die nose throughdie exit 9, with the lumen fluid filling the inner diameter of thefiber.

FIG. 2 illustrate the die nose used for horizontal fiber spinning. Thesolution is introduced to circular inlet 13 from the crosshead die andis transported to die exit 21. Lumen fluid is introduced to the die noseat inlet 12, and exits at the die exit. Heater 15 maintains the solutionin a fluid form. Temperature sensor 16 is used with temperaturecontroller to maintain heater at a determined temperature above theseparation temperature of the solution. Die tip 22 penetrates dienose/cooling bath insulator wall 19 and contacts cooling bath fluid 7held in cooling bath trough 20. Gel membrane hollow fiber 18 exits thedie tip through die exit 21, with the lumen fluid filling the innerdiameter of the fiber.

For vertical extrusion, the die tip is positioned so that the exitinggel fiber dos not pass through an air gap before contacting the coolingbath. A preferred position has approximately 1.6 millimeter ({fraction(1/16)} inch) of the die submerged as represented in FIG. 1. Forhorizontal fiber spinning, the die is firmly positioned against aninsulated surface as shown in FIG. 2. The die Up penetrates through anopening having a liquid-tight seal in the insulator. A trough forcooling liquid flow is placed in a recess in the opposite side of theinsulating seal, in a manner that will maintain the die nose outlet in asubmerged condition. The trough may be permanently fixed or retractable.The trough comprises a longer length of a depth, and a shorter length ofless depth, which butts against the insulator in the recess. Optionally,the trough can be of a single depth with for example, pumping means toremove overflow cooling fluid. Cooling liquid flows in the trough andoverflows the region of the trough of lesser depth, keeping the die noseoutlet submerged with a flow of cooling liquid. Optionally, the troughmay be placed to allow a small flow of cooling liquid between the troughend and the insulator surface.

Although PFA and POLY(PTFE-CO-PFVAE) are similar in chemical structure.POLY(PTFE-CO-PFVAE) was surprisingly different than PFA in terms ofprocessability. PFA tended to quench very fast, possibly due to itshigher melting temperature. Consequently, with submerged extrusion, itwas very difficult to spin at a rate much higher than 40-50 fpm unlessthe lumen fluid was controlled to have a temperature between 260°C.-280° C. Since the lumen fluid would tend to boil at this temperature,spinning at a higher rate was very difficult. Under optimal conditions,the maximum spinning rate of PFA was around 24.4 meters per minute(mpm), (80 feet per minute (fpm)). Probably because of its slightlylower melting point, POLY(PTFE-CO-PFVAE) did not quench as fast.Spinning at could be done at 55.9 mpm(180 fpm). POLY(PTFE-CO-PFVAE)fibers also appear mechanically stronger than PFA, The gel fiber ordried, extracted fiber could be stretched longitudinally, resulting insignificant increase in permeability.

Cooling Bath

The cooling bath lowers the temperature of the extruded fiber to belowthe upper critical solution temperature to cause phase separation. Thebath liquid can be any liquid having a boiling point high enough toprevent bubbles from forming on the fiber exiting the die, and notadversely affecting the surface pore forming process. The bathtemperature can be from 25° C. to 230° C., with a preferred range being50° C. to 150° C.

The bath liquid can be any liquid that does not boil at the coolingtemperature, or at the point where the heated extrudate enters thecooling bath, or interact with the fiber to cause a skin to form, or todissolve or swell the polymer at the cooling bath temperature. Examplesof preferred liquids are dimethylsilicone oil and di-octyl pthalate.Other di-substituted pthalates may be used.

Extraction and Drying

The gel fiber is then introduced into a liquid extraction bath of aliquid that will remove the solvent without substantially softening,weakening, or dissolving the fiber. Suitable extraction solvents includedichlorofluorethane, HCFC-141b, 1,1,2 trichlorotrifluoroethylene (Freon®TF, DuPont), hexane or similar. Extraction is usually done at from about20° C. to about 50° C. to minimize the effect of the extracting liquidon the fiber. The extracted fiber is dried under restraint to preventshrinkage, as on a cylindrical core, at from 20° C. to 50° C.Optionally, the fiber is then heat set at 200° C. to 300° C.

The advantage of the submerged extrusion method is that it can producehollow fiber membrane continuously in practical lengths. Perfluorinatedthermoplastic hollow fiber membranes made by prior art methods breakeasily during extrusion and practical lengths cannot be collected. Themembranes produced by the submerged extrusion method have high surfaceporosity and good permeability. FIGS. 5 and 6 show the inner and outersurfaces respectively for a fiber of Example 1, sample #3. The innersurface has an unskinned surface consisting of nodules. The outersurface is made up of fibrous-like oriented structures. FIGS. 7 and 8show the inner and outer surfaces respectively of a fiber of Example 1,Sample # 8. The inner surface is made up of fiber-like structures in awhorl-like pattern, and the outer surface is primarily made up oforiented fiber-like structures. FIGS. 9 and 10 show the inner and outersurfaces respectively of a fiber of Example 5. Both surfaces are highlyporous, with no smooth skin regions. These Figures illustrate varioushighly porous or skinless surfaces that can be produced in a continuousprocess by the submerged extrusion method. It can be appreciated thathigh surface porosity of the skinless membranes of the present inventionwill be less likely to become plugged up or fouled by particulatesduring a filtering operation. This will result in longer and moreeffective operation of the membrane.

FIG. 3 illustrates a typical process for vertical spinning to producethe hollow fibers of the invention. The polymer/solvent paste-likemixture is introduced into a heated barrel extruder 31 through inlet 32,by means of a pumping system 47, for example, a progressive cavity pump.A solution is formed is formed in the heated barrel of extruder 31.Extruder 31 conveys the heated solution through conduit 33 into meltpump 34 that meters the solution, and then through conduit 35 to crosshead die 36. Optionally, the solution is conveyed from extruder 31through conduit 33 into melt pump 34, and then through conduit 48 tosolution filter 49, and then through conduit 35 to cross head die 36.

The solution passes through the crosshead die 36 and into the die nose 1where the solution is formed into a hollow fiber shape. The lumen fluidis introduced from die mandrel 38 to the inner diameter of the hollowfiber solution exiting from the die. The lumen fluid is supplied to diemandrel 38 by means of lumen fluid supply means 46.

For vertical fiber spinning, the solution with lumen fluid is extrudedfrom die nose 1 vertically with no air gap into cooling bath fluid 7contained in cooling bath 41 where the solution is cooled to effect themicrophase separation of polymer and solvent into a gel membrane hollowfiber 8. The gel membrane hollow fiber 8 is guided through the coolingbath 41 by guide rollers 43 and is removed from the cooling bath 41 byGodet rolls 44. The gel membrane hollow fiber 8 is removed from theGodet rolls 44 by cross winder 45.

FIG. 4 illustrates a typical process for horizontal spinning to producethe hollow fibers of the invention. The polymer/solvent paste-likemixture is introduced into a heated barrel extruder 31 through inlet 32,by means of a pumping system 47, for example, a progressive cavity pump.A solution is formed is formed in the heated barrel of extruder 31.Extruder 31 conveys the heated solution through conduit 33 into meltpump 34 which meters the solution, and then through conduit 35 to crosshead die 36. Optionally, the solution is conveyed from extruder 31through conduit 33 Into melt pump 34, and then through conduit 48 tosolution filter 49, and then through conduit 35 to cross head die 36.

The solution passes through the crosshead die 36 and into the die nose 1where the solution is formed into a hollow fiber shape. The lumen fluidis introduced from die mandrel 38 to the inner diameter of the hollowfiber solution exiting from the die. The lumen fluid is supplied to diemandrel 38 by means of lumen fluid supply means 46.

For horizontal fiber spinning, the solution with lumen fluid is removedfrom the die nose 1 through the die/cooling bath insulator wall 19 withno air gap into cooling bath fluid 20 contained in cooling bath 51 wherethe solution is cooled to effect the microphase separation of polymerand solvent into a gel membrane hollow fiber 18.

The gel membrane hollow fiber 18 is guided through the cooling bath 51by guide rollers 43 and is removed from the cooling bath 51 by Godetrolls 44. The gel membrane hollow fiber 18 is removed from the Godetrolls 44 by cross winder 45.

Solvent is then removed from the gel fiber by extraction with a solventthat will not significantly weaken or deleteriously affect the hollowfiber membrane. The fiber is then dried under restraint to minimizeshrinkage. Optionally, the fiber may be stretched in the longitudinaldirection. Optionally, the fiber may be heat set. The resultingperfluorinated thermoplastic porous hollow fiber membranes of thepresent invention have porous surfaces on inner and outer surfaces andat least one surface having no skin. The membranes have flow propertiescharacterized by flow times (described below) of less than 3000 seconds.

Contactor Membranes

In the contactor embodiment of the invention, the same hollow fibermembrane manufacturing process as described for porous membranes isused, with some differences in the operating ranges of the processparameters.

The percent solids of the fiber spinning solution is from about 25% toabout 40%, with a preferred range of from about 28% to about 33%. Thepaste is metered into the heated mixing zone of a conventional twinscrew extruder and heated to a preferred temperature of about 270° C. toabout 320° C., with a more preferred range of 285° C. to 310° C. Infiber extrusion, wall thicknesses of from 50 microns to 250 microns,preferably 100 microns to 150 microns, will give a useful fiber. Outerdiameter/inner diameter ranges typically are 800-1200/400-700 microns.Spin line take-up rates are dependent on the fiber dimensions andextrusion rate. Rates of from approximately 20 to approximately 200 feetper minute can be used, with a preferred rate being approximately100-150 feet per minute.

During fiber extrusion the inner diameter of the die is filled with acontinuous flow of liquid to prevent the fiber lumen from collapsing.Careful control of the lumen liquid flow rate is required to preventuncontrolled variations in fiber dimensions. The liquid should have aboiling point high enough so that boiling will not occur in the die orthe extruded fiber. This can cause bubbles in the lumen and fiberbreakage. The lumen liquid should not affect the fiber inner wall in away that will cause the inner surface to densify. As, for example, bycausing coagulation of the heated solution at the lumen liquid-innerwall contact interface, or by extracting solvent from this interface andincreasing the surface polymer concentration. The lumen liquid can bemetered into the die at room temperature, or preheated to a temperatureof up to about 250° C., with a preferred range of 215° C. to 235° C.

The die is comprised of a standard crosshead die, to which is attached adie nose. The die has two temperature control zones. The crossheadportion of the die is kept at 270° C. to 320° C., with preferredtemperature range being 290° C. to 310° C. The die nose, whichencompasses the die outlet, is controlled separately to a range of 290°C. to 350° C., preferably to 320° C. to 340° C. The die nose heated zonebriefly raises the solution temperature to near or above the boilingpoint of the solvent.

The cooling bath lowers the temperature of the extruded fiber to belowthe upper critical solution temperature to cause phase separation. Thebath liquid can be any liquid having a boiling point high enough toprevent bubbles from forming on the fiber exiting the die, and notadversely affecting the surface pore forming process. The bathtemperature can be from 25° C. to 230° C., with a preferred range being50° C. to 150° C.

Characterization Methods

Flow Rate Test

Two strands of fiber are loops to fit into a ¼″ polypropylene tubingabout 1″ long. A hot melt gun is used to force hot melt glue through theopen end of the tubing to pot the fibers. Normally, the glue does notfill up all the spaces between the fibers. To complete the potting, hotmelt glue is applied to the other end of the tube. The length of thefibers, from the end of the potting to the loop, should be about 3.5centimeters. After the hot melt glue solidifies, the tubing is cut toexpose the fiber lumens. The fiber OD is measures under a microscope.The tubing with the fiber loop is mounted into a test holder. Isopropylalcohol (IPA) is poured into the holder, the holder sealed, and gaspressure is set to 13.5 psi. The time interval to collect a set amountof IPA permeate is recorded.

Sample Calculations

IPA Flow RATE=V/(T*π*OD*N*L)

IPA FlowTime (FT)=seconds to collect 500 ml IPA permeate; calculatedfrom the time measured to collect a convenient volume from the set-updescribed. where;

v=volume of permeate

T=time

OD=outside diameter of fiber

N=number of fibers

L=total length of one strand of exposed fiber

Visual Bubble Point

The potted fiber loop is mounted in a bubble point test holder. The loopis submerged in a glass container of IPA. Air pressure is slowlyincreased in the lumen of the fibers. The pressure at which the firstbubble appears at the outer surface of the fibers is registered as thevisual bubble point.

Mean Bubble Point

A method similar to ASTM F316-80 was used to determine mean bubblepoint. A curve of airflow through a potted sample venus pressure wasplotted for a dry sample and for the same sample wetted with IPA. Themean bubble point is the pressure at which the wet airflow is one halfthe dry airflow.

Scanning Electron Microscopy Images

Samples of hollow fiber membrane are soaked in isopropyl alcohol or amixture of isopropyl alcohol and water, approximately 50% by volume. Thewetted sample is then soaked in water to replace the alcohol. Thewater-wetted sample is held by a tweezer and dipped in a container ofliquid nitrogen. The sample is then removed and quickly snapped bybending using a pair of tweezers. Approximately 2 millimeter cut sampleis fixed to a sample stub with conductive carbon paint (Structure ProbeInc. West Chester Pa.). Microscopy is done with an ISI-DS130c scanningelectron microscope (International Scientific Instruments, Inc,Milpitas, Calif.). Digitized images are acquired by a slow scan framegrabber and stored in .TIF format.

EXAMPLE 1

Pellets of Hyflone POLY(PTFE-CO-PFVAE) 620(Ausimont) was mixed withHaloVac 60 from Halocarbon Oil Inc. to produce a paste of 18% by weight,which was fed by a Moyno pump into a Baker-Perkins MPC/V-30; L/D=13twin-screw extruder operating at 200 RPM in the horizontal fiberspinning mode.

Extrusion and run conditions are shown in Tables 1 and 2 below. A Zenithmelt pump was used to meter the melt into a hollow fiber die. The dieannulus was about 400μ. Heated Halocarbon oil 1000 N was used as lumenfluid to maintain the hollow portion of the fiber. The melt pump and thelumen fluid pump were adjusted to produce a fiber with about 200μ walland 500μ lumen.

The bath liquid was dioctyl pthalate. After centering of the lumenneedle, the die was submerged under the quench liquid for about{fraction (1/16)}″ and the fiber was taken up by a set of Godet rolls.The fiber was extracted with Genesolv® 2000, Allied-Signal, Morristown,N.J., dried and then annealed at 275° C. Fiber Characterization data aregiven in Table 3.

TABLE 1 Extruder Barrel temperatures Temperatures (° C.) Melt (° C.)Sample Zone Zone Zone Zone temperature Die Die # 1 2 3 4 (° C.) bodyNose 1 230 290 285 285 285 280 310 2 230 290 285 285 285 275 310 3 230290 285 285 285 275 310 4 230 290 285 285 285 275 310 5 230 290 280 280277 280 310 6 230 290 280 280 277 280 310 7 230 290 280 280 277 280 3108 230 300 280 280 285 280 310

TABLE 2 Lumen Cooling Take-up pump bath Sample rate rate Temperature #(fpm) (rpm) (° C.) 1 100 20  55 2 100 25 100 3 130 25 100 4 130 15 100 5100 30 100 6 100 35 100 7 100 45 100 8 200 25 100

TABLE 3 Outer Wall Visual IPA Mean IPA Flow Sample diameter thicknessbubble bubble Time # Microns microns point (psi) point (psi) (sec) 1 940191 16 39.5 1396 2 914 184 14 37.3 1028 3 826 165 15 37.6 916 4 749 21019 40.5 1467 5 1054  178 14 27.3 933 6 1080  172 10.5 27.3 783 7 1118 140 10 37.9 788 & 826 203 12 29 1295

EXAMPLE 2 Effect of Stretchinq

A fiber produced in manner similar to those in Example 1., from an 18%solids solution of POLY(PTFE-CO-PFVAE) in HaloVac 60 was extracted,stretched 100% and annealed at 275° C. The results in the Table belowshow the improvement in permeability due to stretching.

Unstretched Stretched OD microns 851 723 ID microns 381 343 Wall microns229 191 IPA visual bubble point (psi) 15 10 IPA mean bubble point (psi)38 23 IPA flow time (sec) 2000 835

Example 3. Blends ofpoly(tetrafluoroethylene-co-perfluoro(methylvinylether)) (A) andpoly(tetrafluoroethylene-co-perfluoro(propylvinylether)) (B)

Hollow Fiber membranes were spun in a manner similar to Example 1 withthree blends of A and B. The total solids in the paste was 20%. Take-uprate was 50 feet per minute. The cooling bath was dioctyl pthalate at 85(° C.). Fiber spinning conditions are given in Tables 4 and 5. Membranecharacterization data are given in Table 6.

TABLE 4 Extruder Barrel temperatures Temperatures (° C.) Melt (° C.)Blend Zone Zone Zone Zone temperature Die Die A/B 1 2 3 4 (° C.) bodyNose 90%/ 200 295 295 295 295 285 300 10% 80%/ 200 295 295 295 295 285300 20% 20%/ 200 295 295 295 295 285 310 80%

TABLE 5 Lumen Cooling Take-up pump bath rate rate Temperature Blend A/B(fpm) (rpm) (° C.) 90%/10% 50 10 85 80%/20% 50 10 85 20%/80% 50 10 85

TABLE 6 Visual IPA Mean IPA Outer Wall bubble bubble Flow diameterthickness point point Time Blend A/B Microns microns (psi) (psi) (sec)90%/10% 953 130-279 71 45 1318 80%/20% 914 130-279 16 40 1194 20%/80%927 130-279 12 44 1362

EXAMPLE 5 Poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP)

The process conditions for spinning FEP hollow fiber membranes were thesame as for the blend membranes of Example 4, except for the barreltemperature and the die temperatures. A 20% solids paste was used. Eventhough the melting point of FEP, about 258° C., is much lower thanpoly(tetrafluoroethylene-co-perfluoro(methylvinylether)), it wassignificantly more difficult to dissolve than either PFA orPOLY(PTFE-CO-PFVAE). To spin FEP, the barrel temperature had to beraised from 295° C. to 305° C. and the die nose temp. from 300° C. to320° C. The membrane properties of FEP hollow fiber membranes spun inthis example were; IPA visual BP 12.6 psi., mean BP 40 psi. and flowtime 1593 seconds.

Comparative Examples

Hollow fiber membranes were produced using a process similar to that ofExample 1 of U.S. Pat. No. 5,032,274. A 19% solution ofpoly(tetrafluoroethylene-co-perfluoro(propylvinylether)) in HaloCarbonOil 56 was extruded into a 150° C. cooling bath of Halocarbon 1000 N, apoor solvent for the polymer. Extruder barrel temperatures were 150° C.,285° C., 260° C., 280° C., for Zones 1-4. The melt temperature was 308°C. The extruder was run at 300 RPM. The lumen fluid pump was run at28-30 RPM.

A short portion of hollow fiber membrane was produced with no solventcoating at a take-up rate of 55 feet per minute. The air gap between thedie exit and the cooling bath surface was 0.25 inch. The fiber had an ODof 1500 microns and a wall thickness of 250 micron. IPA flow time was42,735 seconds.

A short portion of hollow fiber membrane was produced using the solventcoating method at a take-up rate of 50 feet per minute. HaloCarbon 56was co-extruded with the fiber. The air gap was 0.50 inch. The OD was2000 microns and the wall thickness was 250 microns. IPA flow time was3315 seconds.

These examples illustrate that fibers produced by the earlier method arenot able to produce fiber with the desirable property of low flow times.Low flow times relate to higher membrane permeability and shorterfiltration times.

EXAMPLE 6

In this example a skinless contactor hollow fiber membrane made from the30% polymer solution described above was compared to a skinned fibermade from a 30% polymer solution according to the method of MCA 422, ournumber, serial number not yet assigned. A gas mixture containing ozone,a highly water soluble gas, was contacted with water using thesemembranes.

A skinless contactor hollow fiber membrane was made by the followingmethod. Powdered Hyflon MFA (Ausimont, Thorofare, N.J.) was mixed withHaloVac 60 from Halocarbon Oil Inc Halocarbon Products Corporation,River Edge, N.J. to produce a paste of 30% polymer content which was fedby a Moyno (Springfield, Ohio) melt pump into a Baker-Perkins (Saginaw,Mich.) twin-screw extruder. The extruder barrel temperatures were set tobetween 180-288° C. A Zenith (Waltham, Mass.) melt pump was used tometer the melt into the special hollow fiber die mentioned above. Thedie annulus was about 300 micron. Halocarbon oil, Halovac-60 was fed byZenith pump in the lumen to maintain the hollow portion of the fiber.The melt pump and the lumen oil pump were adjusted to produce a fiberwith about 200 micron wall and 700 micron lumen. The temperature of thebath liquid, mineral oil, was set to 70° C. After centering of the lumenneedle, the die was submerged horizontally under the quench liquid andthe fiber was taken up by a set of Godet rolls running at 100 feet perminute. The fiber was extracted by (1,1 dichloro1-fluorethane,(Florocarbon 141b, Genesolve 2000 Allied-Signal, N.J.) and driedsubsequently. This fiber had an IPA visual bubble point of >40-50 psiwith an IPA flow time 12,000 sec. The intrusion pressure for thesefibers was 8-10 psi.

Each contactor was installed onto the test stand depicted in the FIG.11. Deionized water at 23° C. temperature and a pH of 6.2 was pumpedthrough the lumen side of the membranes at varying flow rates. Waterfrom the deionized water system (not shown) enter through valve 142 withbypass valve 141 closed. Pressure gauges 150, 151 measure the water flowpressure drop across the contactor. The ozone contactor 160 was eitherone containing skinned membranes (102698 unit) or one with skinlessmembranes (12798 unit). Ozone gas from a Sorbious Semozon 090.2 HP ozonegenerator was fed at a flow rate of 2 standard liters per minute (slpm)through inlet 130 to the shell side of the contactor unit (160).Contactor gas pressure was measured by pressure gauge 152 and controlledby pressure controller 180. Outlet gas sensor 112 measured outlet ozoneconcentration. The dissolved ozone in the contactor outlet stream wasmeasured by ozone sensor 111. The dissolved ozone in the overflow rinsebath 100 was measured using an Orbisphere Model 3600 dissolved ozonesensor 110. Liquid flow was changed from 3.6 to 20 lpm by adjustingvalve 140 and the inlet liquid pressure. Gas pressure into the shellside of the contactor was adjusted to make the pressure of the gas justlow enough below the pressure of the liquid to prevent the formation ofbubbles in the liquid as gas was transferred to the liquid through themembrane.

FIG. 10 is a plot of dissolved ozone in the outlet water measured inparts per million (ppm) ozone. vs. DI water flow rate in liters perminute for each contactor. The results show that the dissolved ozone inwater decreases with increasing DI water flow rate and that the skinlessfiber contactor dissolves more ozone into the DI water than the ozonecontactor (102698) containing the skinned fiber.

EXAMPLE 7

The skinned and skinless membranes of example 6 were compared in a testwith carbon dioxide, a highly water-soluble gas.

For each contactor used in this Example, a bundle of fibers was made,potted and installed in a cylindrical holder to make a contactor thatseparated the lumen side from the shell or outer side of the fibers.Fiber ID was 500μ and the fiber wall was about 150μ The number of fiberswas about 500 and the length of the module was about 43 cm. Contactorswere used to test for gasification efficiency. In this mode, waterdegassed by a Hoechst Liquid Cel degasser at 20° C. was pumped throughthe fiber lumens. Air containing carbon dioxide was pumped atlow-pressure drop across the shell side of the fibers. For all practicalpurpose, the absolute gas pressure was assumed to be 760 mm Hg. Theozone concentration of the feed and the outlet water was measured atdifferent flow rates.

FIG. 12 shows the results and theoretical predictions based on Leveque'ssolution. The method of data analysis is presented below.

The mass transfer coefficient, K, was calculated by the followingequation:

K=−(Q/A)* ln[C _(out) −C*/C _(in) −C*]

where

C_(out) is the carbon dioxide conc. in output liquid [ppm]

C_(in) is the carbon dioxide conc. in input liquid [ppm]

C* is the equilibrium carbon dioxide conc. at the gas pressure on theshell side [ppm]

Q is the flow rate [cc/s]

A is the membrane area [cm²].

The Sherwood number is calculated as follows:

Sh=K*D/D _(ab)

where

K is the mass transfer coefficient [cm/s],

D is the ID of the fiber [cm] and

D_(ab) is the diffusivity of carbon dioxide in water[cm²/s].

The Graetz or Peclet number is calculated as follows:

Pe or Gr=V*D ²/(L*D _(ab))

Where V is the velocity of flow inside the lumen [cm/s] and L is thelength of the fiber [cm]

The Sherwood and Graetz numbers are dimensionless groups used todescribe heat and mass transfer operations. The Sherwood number is adimensionless mass transfer coefficient, and the Graetz number is adimensionless group that is related to the inverse of the boundary layerthickness.

S. R. Wickramasinghe et al (J. Membrane Sci. 69 (1992) 235-250) analyzedoxygen transport in a hollow fiber membrane contactor using the methodof Leveque. A bundle of porous hollow fiber membranes were used. Theyshowed that a plot of the Sherwood number vs. the Graetz number waslinear at high values of the Graetz number, in agreement withtheoretical predictions. Results at low Graetz number were explained bythe polydisperity of fiber diameters, which affects uniformity of flowthrough the fibers. Their analysis showed that at low Graetz numbers,the average mass transfer coefficient falls below the theoreticalprediction due to uneven flow through the fibers. They concluded thatoxygen mass transfer was unaffected by the diffusional resistance acrossthe membrane. Conversely, one can conclude that a membrane that followsthe prediction of the Leveque theory is porous, because otherwise, theresistance to diffusion would be too high to follow the theory.

The results illustrated in FIG. 12 show that the skinless membranes ofthis example have a low membrane resistance to ozone transport becausethey follow the Leveque equation at high Peclet numbers. In the linearregion, the relationship between the Sherwood number and the Graetznumber is given as Sh=1.64(Gr)^(0.33) for Graetz numbers from betweenabout 5 to about 1000.

What is claimed is:
 1. A hollow fiber porous membrane comprising aperfluorinated thermoplastic polymer having an essentially skinlesssurface on at least one surface and characterized by an IPA flow time ofless than about 3000 seconds to pass 500 ml isopropyl alcohol throughsaid membrane, said membrane formed by an extrusion process using no airgap.
 2. The membrane of claim 1 wherein said membrane is asymmetric. 3.The membrane of claim 1 or 2 wherein the IPA flow time is less thanabout 2000 seconds.
 4. The membrane of claim 1 or 2 wherein the IPA flowtime is less than about 1500 seconds.
 5. The membrane of claim 1 or 2wherein said perfluorinated thermoplastic polymer ispoly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) orpoly(tetrafluoroethylene-co-hexafluoropropylene).
 6. The membrane ofclaim 5, wherein the alkyl of saidpoly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) is propyl,methyl, or blends of methyl and propyl.